Lecture 3 - EE105 Fall 2009 Lecture 3, Slide 1 Prof....

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Unformatted text preview: EE105 Fall 2009 Lecture 3, Slide 1 Prof. Salahuddin, UC Berkeley Lecture 3 OUTLINE PN Junction Diodes (contd) I-V characteristics Reverse breakdown Large-Signal and Small-Signal Models Reading: Chapter 2.2-2.3, 3.4 EE105 Fall 2009 Lecture 3, Slide 2 Prof. Salahuddin, UC Berkeley ln 2 i D A n N N q kT V 1. Current flowing in a semiconductor is comprised of drift and diffusion components. Drift due to electric field Diffusion due to concentration gradient and random thermal motion 2. P-N junction diode: 3. A charged region exists at the junction of a p-n diode 4. This charged region creates an electric field across the junction that in turn creates an built-in potential 5. Built in potential act against diffusion at equilibrium and can help or restrict an external voltage dx dp qD dx dn qD E qn E qp J p n n p tot Review of the last class Three regions of operations: Vd=0 Vd>0 Vd<0 EE105 Fall 2009 Lecture 3, Slide 3 Prof. Salahuddin, UC Berkeley Effect of Applied Voltage The quasi-neutral N-type and P-type regions have low resistivity, whereas the depletion region has high resistivity. Thus, when an external voltage V D is applied across the diode, almost all of this voltage is dropped across the depletion region . (Think of a voltage divider circuit.) If V D < 0 ( reverse bias ), the potential barrier to carrier diffusion is increased by the applied voltage. If V D > 0 ( forward bias ), the potential barrier to carrier diffusion is reduced by the applied voltage. V D + I D EE105 Fall 2009 Lecture 3, Slide 4 Prof. Salahuddin, UC Berkeley PN Junction under Reverse Bias A reverse bias increases the potential drop across the junction. As a result, the magnitude of the electric field in the depletion region increases and the width of the depletion region widens. 1 1 2 D D A si dep V V N N q W EE105 Fall 2009 Lecture 3, Slide 5 Prof. Salahuddin, UC Berkeley Diode Current under Reverse Bias In equilibrium, the built-in potential effectively prevents carriers from diffusing across the junction. Under reverse bias, the potential drop across the junction increases; therefore, negligible diffusion current flows. A very small drift current flows r ( x ) x-qN A qN D a-b V ( x ) x a-b V EE105 Fall 2009 Lecture 3, Slide 6 Prof. Salahuddin, UC Berkeley PN Junction Small-Signal Capacitance A reverse-biased PN junction can be viewed as a capacitor, for incremental changes in applied voltage....
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Lecture 3 - EE105 Fall 2009 Lecture 3, Slide 1 Prof....

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